DC Brushless motors (herein referred to as ‘DCBM’) have been used for a long time in a large range of general applications. Recently, DCBMs have increasingly been used in medical applications to drive small pumps. One such application includes the use of this type of motor in implantable medical devices including, but not limited to, blood pumps and drug infusers.
A DCBM usually comprises a rotor and a stator. The stator often includes three or more phases, wherein at least two of the phases are provided with driving signals to 15 facilitate the rotation of the rotor, while one phase is used to measure the back electromotive force (EMF). It is known in this field of art that the measured back EMF voltage may be used to detect the rotor position in the DC brushless motor. U.S. Pat. No. 4,928,043, to Plunckett et al., describes one example where the rotor position in a DC brushless motor is predicted and anticipated.
Usually the timing of the driving signals provided to the phases in the motor is predetermined with respect to the “typical” fluid dynamic conditions that the motor is expected to encounter. However, any deviation of the external environment from these “typical” conditions reduces the efficiency of the predetermined timing for sending the driving signals to the phases. This is of specific concern in the case of a DCBM being used in an implantable blood pump, if the back EMF control systems fail to correct for hematocrit changes and torque changes necessitated by varying blood temperature and viscosity. There may also be other load factors that can affect the efficiency of the DCBM that include, but are not limited to, the pulsatility of a natural heart. Accordingly, to improve the motor efficiency of DCBM, it is preferable if the motor is continuously tuned so that the firing sequence of the phases matches constantly the varying dynamics characteristics of the operating environment of the pump.
Previously, there have been two prior art methods of measuring back EMF. The first one conducts the EMF measurements based on the OFF periods of the pulse width modulation (PWM). In this case the generated back EMF signal passes from a negative voltage to a positive voltage and allows the so-called “zero crossing” technique to be applied. However, this method generally does not work wherein the PWM is at 100% since there is no OFF time.
The second method includes measuring of the back EMF during the ON periods of the PWM. The generated BEMF signal is always positive and, in theory, should cross half way between the DC bus rail voltages. However, in practice, this often does not occur due to the circuitry and overall DCBM design. Furthermore, this method is generally not efficient across the whole operating spectrum (e.g. low-high speed, or low-high load).
The present invention aims to at least address or ameliorate one or more of the above problems.